Recently, our laboratory has begun to investigate the cross talk between insulin receptor signaling with that of the non-receptor tyrosine kinases Src, Lyn and Fyn. We have found that Fyn null mice display markedly reduced adiposity, increased fatty acid oxidation, enhanced cateholamine-stimulated lipolysis and improved insulin sensitivity. Surprisingly, this mice also gain substantial weight and adipose tissue mass on a high fat diet, yet remain highly glucose tolerant even compared to their lean wild type controls despite marked dyslipidemia. The enhancement of skeletal muscle and adipose tissue oxidation occurred concomitant with increase AMPK phosphorylation on a subunit threonine 172 (activation site phosphorylation) and with increased acetylCoA carboxylase phosphorylation on its negative regulatory site. The increased rate of lipolysis resulted from increased phosphorylation of perilipin and activation site phosphorylation of hormone sensitive lipase. Although the conventional Fyn null mice display a remarkable improvement in energy utilization and insulin sensitivity, these data do not address whether the loss of Fyn function manifests through acute alterations in signaling or through developmental adaptation. Moreover, there are three splice variants of Fyn that appear to have distinct functional and tissue-specific distribution patterns. Thus, the analysis of the conventional Fyn null mice cannot address these issues. We therefore propose in this R21 application to determine whether these effects occur in a cell autonomous versus non-autonomous manner by generating isoform tissue-specific Fyn transgenic expressing mice and inducible tissue-specific Fyn knockout mice. We then propose to use knock-in technology to generate isoform specific Fyn expression in skeletal muscle and adipose tissue. These animals will be examined by detailed analysis of the molecular signaling pathways regulating metabolic signaling in relationship to whole body integrative system physiology. Diabetes, obesity and insulin resistant states can all be characterized as defects in the body's ability to adjust for differences in energy consumption and output. We have found that the enzyme Fyn regulates fatty acid oxidation only in the fasted state but not in the fed state. This means that it may be possible to modulate Fyn activity and thereby increase fatty acid oxidation and energy expenditure (e.g.: decrease obesity). Thus, this project is focused on developing and characterizing mouse models to directly test the integrative physiologic consequences of altered Fyn function. [unreadable] [unreadable] [unreadable]